iSetSquare 4+

Otherwise I have no complain.

Surprisingly accurate. When I go smoothly CW, measuring out 180 degrees in about 4 seconds, I often get within 1 degree. When I go CCW, however, it seems to be no better than 2 degrees, and often 3 or 4 degrees off. This is still around 2% accuracy, which is very good. I get this kind of error routinely, even if I let the gyro stabilize to minimize drift. It thus might be necessary to have two calibration values: one for CW and one for CCW turns.

I am also having troubles seeing that the calibration I'm setting is doing anything. While I really like the concept of how you calibrate it, it is a unclear when exactly it is using the calibration for its output. Since calibrating depends on measuring the angle with calibration off, it should be very straight-foward to have calibration on or off in the settings, and even to have a color indication in the goniometer view itself so that we know calibration is or isn't taken into account.

Other than that, I only have one small feature request: present a drop-down menu of angle formatting choices, since there are so many to toggle through.

I must say the accuracy is excellent. I used it to get an angle for my compound mitter saw. I drew the line on the board at the angle I needed and use this app as a protractor. Then set up my saw and put the board on it. The line and the saw where exactly the same.
I haven't used it in to many situations yet but if ever lets me down I will change my review. But I thinks it's worth money.
I even tried it where I new it wasn't going to work like a compass just to see if you needed any special external signal and it still worked .This is important to me cause I work in a Steel Mill in basements and places with no reception at all.